#lang sicp
(define (make-from-mag-ang mag ang)
  (define (dispatch op)
    (cond ((= op 'mag) mag)
          ((= op 'ang) ang)
          ((= op 'real) ...)
          ((= op 'imag) ...)
          (else (error "unknown operation! -- MAKE-FROM-MAG-ANG" op))))
  dispatch)